Such regenerator vessels are for example described in U.S. Pat. No. 4,435,282. This publication describes a vessel comprising at its lower end a bubbling fluidised bed of catalyst in which the combustion of coke present on the catalysts to be regenerated takes place.
A disadvantage of the bubbling bed regenerator as described in U.S. Pat. No. 4,435,282 is that they are troubled with stagnant beds, due to poor catalyst flow patterns, and regeneration gas bypassing, due to the formation of large bubbles within the bubbling dense bed.
An alternative to the bubbling bed regenerator is the so-called “High Efficiency Regenerator” (HER) design as described in EP-A-610186. The regeneration is performed in a fast-fluidised bed for most of the coke combustion and a dilute phase transport riser for some CO combustion. Regenerated catalyst is collected in a bubbling dense bed for reuse and for recycle to the coke combustor. The regeneration of catalysts is more efficient than in the bubbling bed regenerator. As a result the catalyst inventory required in a FCC unit having such a regenerator can be smaller than the inventory of a FCC unit having a bubbling bed regenerator This is advantageous, for example because such a FCC unit can refresh or change its catalyst inventory in a more efficient manner.
A disadvantage of the “High Efficiency Regenerator” (HER) design as for example described in EP-A-610186 is their mechanical and operational complexity. For example the regenerator vessel consists of two stacked vessels comprising three different fluidised bed regimes.
GB-A-769818 discloses a vessel, wherein both stripping and regeneration is performed. Catalyst from the FCC reactor is supplied to an inner vessel zone wherein catalyst is stripped to obtained spent catalyst. The spent catalyst can flow via openings from inner vessel space to the outer vessel space. In the latter space regeneration takes place.
U.S. Pat. No. 5,198,397 describes a regenerator wherein the spent catalyst is directly introduced in a fast-fluidised bed zone located centrally in a regenerator vessel. The required temperature for achieving a sufficient combustion of the coke from the catalyst is achieved by mixing the spent catalyst with part of the regenerated catalyst at the catalyst inlet. The partly regenerated catalyst then flows to a second fluidised bed located in the annular space between the centrally located fast-fluidised bed zone and the regenerator vessel wall. Disclosed is a regeneration process wherein partial combustion takes place in the fast-fluidised bed and total combustion takes place in the second fluidised bed. In order to avoid after burning in the upper part of the regenerator vessel, when the effluent gasses of both fluidisation zones meet, a special cyclone arrangement is proposed.